This proposal utilizes the cellular slime mold, Dictyostelium discoideum, to consider two problems of post-transcriptional regulation of gene expression: 1) What is the function of poly(A)? and 2) What are the determinants of mRNA stability? We postulate that poly(A) tails do not regulate mRNA stability, but rather that they have a role in protein synthesis that is mediated by the cytoplasmic poly(A)-binding proteins. The first half of this proposal addresses this hypothesis by: a) determining the translational efficiency and poly(A) tail length of individual mRNAs in growing and developing cells, b) characterizing specific binding properties of the poly(A)-binding proteins and c) testing whether antibodies which recognize the poly(A)-binding proteins can inhibit protein synthesis in vitro. The experiments of the second half of this proposal will consider alternate explanations for the regulation of mRNA stability. Two sets of cDNAs which encode stable or unstable mRNAs, respectively, have been cloned and will be used to compare the properties of these two classes of mRNA. We will determine whether mRNA stability correlates with translational efficiency or with the presence of specific 5' or 3' untranslated sequences, transcribed repetitive sequences, or poly(A). In addition, we will determine whether the half-lives of specific mRNAs are differentially regulated during the Dictyostelium life cycle.